The present invention relates to high porosity propped fractures and methods of creating high porosity propped fractures in portions of subterranean formations. More particularly, the present invention relates method of using weighting agents in fracturing fluids used to create high porosity propped fractures.
Subterranean wells (such as hydrocarbon producing wells, water producing wells, and injection wells) are often stimulated by hydraulic fracturing treatments. In hydraulic fracturing treatments, a viscous fracturing fluid, which also functions as a carrier fluid, is pumped into a portion of a subterranean formation at a rate and pressure such that the subterranean formation breaks down and one or more fractures are formed. Typically, particulate solids, such as graded sand, are suspended in a portion of the fracturing fluid are then deposited in the fractures. These particulate solids, or “proppant particulates,” serve to prevent the fractures from fully closing once the hydraulic pressure. By keeping the fracture from fully closing, the proppant particulates aid in forming conductive paths through which fluids may flow.
Commonly used proppant particulates generally comprise substantially spherical particles, such as graded sand, bauxite, ceramics, or even nut hulls. Generally, the proppant particulates are placed in the fracture in a concentration such that they formed a tight pack of particulates. Unfortunately, in such traditional operations, when fractures close upon the proppant particulates they can crush or become compacted, potentially forming non-permeable or low permeability masses within the fracture rather than desirable high permeability masses; such low permeability masses may choke the flow path of the fluids within the formation. Furthermore, the proppant particulates may become embedded in particularly soft formations, negatively impacting production.
The degree of success of a fracturing operation depends, at least in part, upon fracture porosity and conductivity once the fracturing operation is stopped and production is begun. Traditional fracturing operations place a large volume of proppant particulates into a fracture and the porosity of the resultant packed propped fracture is then related to the interconnected interstitial spaces between the abutting proppant particulates. Thus, the resultant fracture porosity from a traditional fracturing operation is closely related to the strength of the placed proppant particulates (if the placed particulates crush then the pieces of broken proppant may plug the interstitial spaces) and the size and shape of the placed particulate (larger, more spherical proppant particulates generally yield increased interstitial spaces between the particulates). Such traditional fracturing operations tend to result in packed fractures that have porosities ranging from about 26% to about 46%.
One way proposed to combat problems inherent in tight proppant particulate packs involves placing a much reduced volume of proppant particulates in a fracture to create what is referred to herein as a partial monolayer or “high porosity” fracture. In such operations the proppant particulates within the fracture may be widely spaced but they are still sufficient to hold the fracture open and allow for production. Such operations allow for increased fracture conductivity due, at least in part, to the fact the produced fluids may flow around widely spaced proppant particulates rather than just through the relatively small interstitial spaces in a packed proppant bed.
While this concept of partial monolayer fracturing was investigated in the 1960's, the concept has not been successfully applied for a number of reasons. One problem is that successful placement of a partial monolayer of proppant particulates presents unique challenges in the relative densities of the particulates versus the carrier fluid. Another problem lies in the fact that placing a proppant that tends to crush or embed under pressure may allow the fracture to pinch or close in places once the fracturing pressure is released. Still another problem is that particulates that could be carried in a fluid to potentially produce a high porosity fracture were then unable to support the load from the formation once the fracturing pressure was released. Attempts to solve these problems have heretofore been unsuccessful.